Work in progress is concerned with: (1) Characterization of mutants of E. coli which are fully sensitive to mecillinam (F1 1060) at 30 degrees but resistant at 42 degrees. (2) Identification of the biochemical lesion caused by mecillinam. (3) Characterization of the genetic lesion in a temperature-sensitive mutant of E. coli deficient in lipoprotein.